Which should I buy? Except for the processor and the color 
, they look identical to me.
I don’t understand that on Canadian Amazon Ar300M is cheaper.
All depends on what you want to use it for.
The MT300N v2 is cheap, but uses proprietary wifi drivers that are not supported by OpenWrt/LEDE.
It has 128MB ram and 16MB flash and a relatively slow processor.
The AR300M is now replaced by the AR300M-16 and AR300M-lite.
All 3 of the AR300Mx models have the same faster processor, 128MB ram and 16MB flash. The original AR300M also had an additional 128MB NAND flash chip but this was not really needed and not easy to use as well as being an expensive chip.
All three AR300Mx models are fully open source and fully supported in OpenWrt/LEDE
For typical light travel router use the MT300N v2 is fine.
If you want to push the device hard for other uses, eg heavy video streaming, media sharing, or IofT stuff, the Ar300Mx is in my opinion a far better choice.
Internally the AR300Mx and the MT300Nv2 are very different.